1. Field of the Invention
This invention relates to a pulse width modulation apparatus and is applicable to, for example, a laser-beam printer.
2. Description of the Related Art
Laser-beam printers have been widely used as printers for printing arbitrary characters and graphics with high quality, in which output information corresponding to characters and graphics is written onto a photoconductive drum by a laser beam and then an image written on the photoconductive drum is printed on a paper by means of electrophotography.
In general, as shown in FIGS. 1 to 4, this kind of laser-beam printer has a built-in pulse generator for controlling the rising and falling timing of an output pulse. For example a laser pulse DO, which rises at intervals of a pulse cycle T, is let fall according to the timing specified in setting data, thus adjusting the pulse width to express the tone.
In other words, the pulse generator decodes 8-bit selection data to specify a falling point in 255 (=eighth power of 2 minus 1 ) unit pulses P1 to P255, which constitute a maximum output pulse width PW0 (pulse width set when a laser beam is output over the entire duration of a pulse cycle T), and thus provides an output pulse of an arbitrary pulse width (FIG. 1).
A falling point of a laser pulse DO is specified by selecting one of delay outputs provided by 255-stage delay elements connected in series with one another via output terminals thereof. For selection of a delay output, programmable delay circuits are usually used.
When an attempt is made to specify a falling point of an output pulse, which is generated at intervals of a pulse cycle T, using the programmable delay circuits, a certain period of time At is required to decode selection data. Despite the absence of an output pulse, an output pulse may be supplied for a certain period of time .DELTA.t from the start of each duration (FIG. 2). On the contrary, despite the presence of an output pulse, no output pulse may be supplied for a certain period of time .DELTA.t (FIG. 3).
For effective use of a maximum output pulse width PW0, multi-stage delay circuits each having 255-stage delay elements connected in series with one another are lined in parallel with one another. The multi-stage delay circuits are operated complimentarily, thus preserving the time required for decoding. This idea, however, requires multi-stage delay circuits having the same performance, which leads to an increase in the number of elements or power consumption.
In the foregoing kind of laser-beam printer, a tone is expressed depending on the pulse width of an output pulse. When an output pulse having a large pulse width is succeeded by an output pulse having a small pulse width (FIG. 4), the output pulse having a short pulse width is incorrectly seen as part of the output pulse having a large pulse width. Thus, the tone cannot be exhibited correctly.
Further, when the pulse width modulation apparatus using an RS-FF circuit (reset/set flip-flop circuit) is employed in digital copying machines and laser-beam printers, an attempt to reproduce image gradation with higher fidelity requires generation of an output pulse having a narrower width than the control pulse, or generation of two successive output pulses with a small gap left therebetween.
Stated otherwise, an attempt to generate an output pulse having a narrower width than the set pulse and the reset pulse applied to the RS-FF circuit has raised the problem that there occur an overlap of durations in which the set pulse and the reset pulse both take the logic "H" level, thus making the RS-FF circuit fail to operate normally.
Also, an attempt to generate output pulses having a wider width in succession than the set pulse and the reset pulse applied to the RS-FF circuit has raised the problem that spans in which the set pulse and the reset pulse take the logic "H" level overlap with each other in the boundary region between the current pulse cycle and the succeeding pulse cycle, thus similarly making the RS-FF circuit fail to operate normally.
This has accompanied a fear that image quality deteriorates in the boundary region between the two output pulses.
While it is contemplated to give priority to either one of the set pulse and the reset pulse, the method cannot solve the two above mentioned problems at the same time. Specifically, if the priority is set to generate an output pulse of narrow pulse width, image gradation deteriorates in the boundary region between adjacent output pulses. On the contrary, if the priority is set to produce a normal output in the boundary region between adjacent output pulses, a narrow output pulse cannot be generated.
In such a pulse width modulation apparatus, however, timing control of rising and falling of the output pulse is performed by such processing as to determine respective complements on several high-order bits and the least significant bit of pulse width setting data, and then determine the sum of those complements, resulting in the problems that an adder occupies the pulse width modulation apparatus to a large extent and the time required for computations is prolonged.
All the reference points of output pulses are set to the start points of clock cycles (i.e. on the left side), when an output pulse with a pulse width over a plurality of cycles is to be generated. A problem arises in that an output image is deteriorated with respect to an original image desired to be printed, and the like. For example, when an output pulse slightly longer than a single clock cycle is intended to be output over three successive clock cycles T1, T2 and T3, a figure represented by a single line in an original image may be reproduced by the pulse width modulation circuit as an image deteriorated in quality due to a space produced between the pulse generated at the first clock pulse T1 corresponding to the left portion of the figure and the pulse generated at the second clock cycle T2 corresponding to the main portion of the figure.
Further, when the reference point is fixed to the intermediate point or end point (i.e., on the right side) of the clock cycle, an image is also deteriorated in quality by the same reason. It is contemplated to generate a delay time longer than the clock cycle by increasing a delay line to prevent the deterioration of the image quality. This, however, leads to an increase in a power consumption and circuit size.
In such a pulse width modulation apparatus, set and reset pulses to be supplied to RS-FF circuit are generated by a programmable delay circuit to provide an output pulse which rises and falls at selected timings.
However, delay time of a delay gate in the programmable delay circuit may fluctuate due to production variance of the integrated circuits or operational environment (operation temperature, source voltage, etc.).
For example, if delay time per one stage of delay gate becomes too long, the delay time of a group of delay gates as a whole should coincide the cycle of a clock pulse becomes longer than the cycle of the clock pulse. As a result, even if a pulse width setting data is provided to make the pulse width of the output pulse to be slightly shorter than the maximum value, it is possible that the pulse width of the output pulse which is actually output may be longer than the ideal pulse width or that the RS-FF circuit is brought into its undefined state because a set pulse for the next cycle is already output by the time at which a reset pulse is output.
On the other hand, if delay time per one stage of delay gate becomes shorter, the pulse width of the output pulse becomes shorter than the ideal pulse width whereby a blank period occurs in the pulse width which should be formed in a manner extending over two clock cycles, resulting in a problem that a stable graduated representation becomes impossible.
Further, since adjusting of the pulse width is also impossible because the delay time of each delay gate once fabricated cannot be adjusted, it may be used only with a certain clock cycle.